1. Field of the Invention
This invention relates to the microwave cooking of food. More particularly, it relates to microwave cooking at higher temperatures than those conventionally obtained. This invention especially relates to the use of liquids whose dielectric properties permit the microwave cooking of foods at temperatures higher than those conventionally obtained.
2. Description of the Prior Art
Microwave ovens for home use have found widespread acceptance. The fact that the cooking and/or reheating of food in the domestic microwave oven is achieved in such relatively short periods of time is probably the principal reason for the high sales of these units in recent years.
Essentially all domestic microwave ovens operate at 2450 MHz. The dielectric properties of food at this frequency parallels those of water which is the principal lossy constituent of food. The absorption of microwave energy by food by the interaction of the dipole water molecule in the microwave energy field results in a localized heating not greatly exceeding 100.degree. C. producing the desired cooking or reheating. Despite the widespread use of microwave ovens, there are several shortcomings to which attention has been directed to increase the utilization of these devices. The uneven absorption of microwave energy by different parts of the same meal placed in the microwave oven results in not all of the separate food components reaching the same temperature. Further, the edges of a particular comestible may be hot while the center may be cold. In addition, the distribution of microwave energy is usually not uniformly distributed throughout the cross section of the oven. Some of these difficulties have been alleviated heretofore by the use of turntables which rotate slowly while the food is subjected to the microwave energy.
Since microwave cooking is dependent upon the absorption of the microwaves which in the case of food means the absorption by water molecules, typical temperatures achieveable in conventional microwave cooking for foods possessing a significant moisture content is approximately 100.degree. C. due to the evaporation of water. Since microwave ovens operate with cold walls, the surface of an article in the oven loses heat to the cool oven walls thereby reducing the surface temperature significantly below that of the temperature obtainable deep within the foodstuff.
Cooking in a microwave is faster than in a conventional thermal oven since the microwave energy has the ability to penetrate deeply into foods and produce heat instantaneously as it penetrates. The conventional thermal oven depends upon the conduction of heat from the surface of the comestible to the interior portions thereof. In microwave cooking the surface temperatures of most moisture-containing foods rarely exceed 100.degree. C. before the inside portion if the food is overcooked, therefore, most foods cooked in a microwave oven lack the brown surface color achieved when using conventional thermal techniques. Thus, breads and pies cooked in a microwave oven have a pale appearance while meats usually have a gray surface appearance when prepared in a microwave oven. The golden brown, crispy texture of roast turkey or chicken, the dark brown appearance of meat roasts and the golden brown crust of pies and cakes are the result of a nonenzymatic browning reactions and/or caramelization. These reactions require temperatures in excess of 125.degree. C. which are not typically obtainable in the microwave oven.
A number of techniques have been utilized heretofore to impart a brown color to comestibles prepared in a microwave oven. In some, browning sauces are utilized to impart the desirable brown color to meats while in others materials are added to cooking utensils which cause an increase in the surface temperature of the comestible being prepared in the microwave oven. U.S. Pat. No. 4,252,832 discloses an aqueous syrup of a melted, carmelized and foamed disaccharide for basting or recipe-inclusion to effect browning of foods cooked in a microwave oven.
By far the more common approach to the problem has been to utilize cooking utensils of various designs whose surfaces include materials designed to be susceptible to heating by microwave radiation thereby causing the surface of the comestible in contact therewith to reach a temperature at which the browning reaction will take place. Among the problems in designing browning utensils are the fact that with some materials susceptible to heating by mccrowave radiation and under some geometric configurations there is no practical upper limit to the temperature to which they will heat so that with prolonged exposure to microwave energy these materials are subject to thermal runaway due to their non-linear dielectric loss characteristics. However, those microwave absorbing materials which exhibit a Curie temperature theoretically have an upper temperature, of about the Curie temperature, which can be attained when subjected to even prolonged microwave radiation. Thus, a number of browning dishes have incorporated one of these desirable class of materials, ferrites, which absorb energy to their Curie temperature, beyond which power absorption decreases and heating does not continue. (See for example, U.S. Pat. Nos. 2,830,162, 4,266,108 and 4,663,506, which disclose ferrite-containing browning utensils.) Thus, foods in heat exchange contact with surfaces containing ferrites will be, when placed in a microwave oven, subject to temperatures sufficiently above 100.degree. C. to cause a browning and/or a crisping of the surface of the comestible.
Other cooking utensils are designed to provide a variety of surface temperatures so that different portions of a comestible receive different rates of heating. Some surfaces may be transparent to microwave energy thereby remaining cool but permitting the portion of the comestible adjacent thereto to become heated by the microwaves passing therethrough. Other surfaces of a utensil may contain microwave lossy materials which can cause these surfaces to become sufficiently hot to promote the browning reaction of the portion of the comestible in heat transfer relationship with that hot surface. U.S. Pat. No. 4,542,271 incorporates into a microwave cooking utensil a heating matrix absorbent to microwave energy composed of a blend of three plastics having magnetite dispersed therein which is said to provide a fast heating time. Other cooking utensils utilize liquids to achieve desirable results. In U.S. Pat. No. 4,439,656, food is packaged in a metal container which in turn is placed in a non-metallic holder slightly larger than the container. The space between the container and holder is filled with a liquid, such as water, which absorbs microwave energy, thereby uniformly generating heat which is transferred to the surfaces of the sides and bottom of the metal container. The use of water, however, does not generate a sufficiently high temperature to promote the browning reaction. U.S. Pat. No. 4,316,070 discloses a moderator for cooking foods evenly in a microwave oven. The moderator is in the form of a plastic bag having a fluid impervious outer layer. The inner surface of the plastic bag has attached thereto a liquid forming layer which may be a layer of a water-soluble surfactant or an absorbent substrate of paper, tissue, cellulosic film or fabric. The liquid used in conjunction with the film forming layer has dielectric properties which influence the bag's ability to moderate or even out the microwave fields. Preferably, the liquid has a dielectric constant above two and a loss tangent below one. Water is the only liquid material exemplified in the patent although suitable dielectics disclosed are water, vegetable oil, ethanol and polyols. The purpose of the surfactant is said to modify the dielectric properties of the liquid by decreasing the dielectric constant and increasing the loss tangent. The film forming layer converts the dielectric fluid placed in contact with the layer into a thin liquid film which surrounds the cooking comestible. By cooking the comestible within the disclosed plastic bag, the microwave energy is moderated prior to its interaction with the comestible so as to cause more even cooking of the comestible.
Much of the prior art has been concerned with providing packages for such food products as pizza, popcorn and fruit and meat pies. These packages are designed to be inexpensive so as to permit their one-time use not only for packaging and consumer-purchase of the comestible, but also to serve as the utensil for microwave cooking of the comestible. Browning of pastry dough while providing even heating of the remaining portion of the comestible is achieved in a variety of techniques involving the use of microwave transparent paper and cardboard together with microwave reflective or microwave shielding materials and microwave lossy materials. These latter materials heat up and radiate or conduct heat to adjacent areas. A variety of the microwave materials, also known as microwave interactive materials, may be used. Because of their dielectric properties, these materials absorb a portion of the microwave energy impinging thereon so that the surface of the material heats up. Examples of suitable microwave interactive materials include metalized layers of polyethylene terephthalate or polyester as disclosed in U.S. Pat. Nos. 4,553,010, 4,590,349 and 4,594,492. Typical examples of commercial packaging utilized for microwave use include the container disclosed in U.S. Pat. No. 4,553,010 for preparing popcorn wherein the container is formed of a bottom panel of paperboard coated with a microwave interactive material of metalized polyethylene terephthalate and the remaining parts of the container are constructed of paperboard which is transparent to the microwave energy. The bottom panel has the microwave reactive material formed on the inner surface thereof in heat transfer relationship with the kernels of popcorn.
A number of prior art patents are devoted to providing a pasteboard or paper container which may be effectively utilized in cooking the pizza contained therein. In U.S. Pat. No. 4,555,605, the package contains a tray, the walls of which are of microwave transparent paperboard and the top of which, also of paperboard, is provided with a microwave interactive layer which converts microwave energy into heat. The pizza is placed on top of the microwave interactive layer and the combination placed in the microwave oven. The pizza filling will be heated directly by the action of the microwave energy while the dough portion of the pizza will be cooked by the heat transferred to it from the microwave reactive film. U.S. Pat. No. 4,592,914 discloses another type of cook-in container for pizza. The container is made from two separate paperboard blanks, one of which forms an outer package while the other forms an inner food supporting tray. The outer package contains a microwave shielding layer of thin aluminum foil to prevent overcooking of the top portion of the pizza. The outer package also contains a section which is removed prior to insertion of the package into the microwave oven to expose several ventilation holes. The inner food supporting tray serves to support the pizza in an elevated position above the bottom wall of the package and at the same time contains a microwave interactive material formed from a metalized layer of polyester which in the presence of microwaves heats up to brown and crisp the crust of the pizza. With a package designed in this fashion, the pizza filling is cooked to a proper temperature while the pizza crust is cooked and browned without becoming soggy.
U.S. Pat. No. 4,626,641 discloses a microwavable container for fruit or meat pies having crusts. The container includes an outer carton formed from a paperboard blank, and an insert formed from a separate paperboard blank containing a microwave transparent area. The outer carton has its entire inner surface laminated with a crisping means constructed from a metalized polyester for converting microwave energy into heat capable of browning and crisping the surface crust of the pie contained within the container. The second paperboard blank is formed into a container to hold the pie which is formed with an upper crust. The insert includes a microwave transparent area in its bottom panel to admit microwave energy into the bottom of the insert and consequently the bottom of the food product held therein. The insert is otherwise shielded on its side and bottom with a microwave reflective material such as aluminum foil to prevent excessive exposure of the food product to direct microwave energy. In this fashion the upper crust of the pie will be brown and crisp while the contents of the pie will be cooked by receiving a controlled amount of microwave energy. Where the pie is made with crust on the bottoms and the sides, the insert design is modified by lining the insert with the same microwave interactive material as was used in the crisping means. In this fashion the insert will provide a crisping and browning effect to the bottom and side crust of the pie while the upper crisping means browns and crisps the upper crust and the pie contents receives appropriate microwave heat for cooking the contents.
U.S. Pat. No. 3,256,101 is directed to materially reducing the time required to heat food in a microwave oven by extracting part of the water from the food and substituting in its place organic liquids having a significantly lower specific heat capacity and latent heat of fusion than water. Typical liquids for this purpose are said to include fatty oils, such as corn oil, cottonseed oil, peanut oil, safflower oil and glycerol. Substitution of an organic liquid for the water significantly reduced the heating time required for heating the food in a microwave oven and also resulted in a more uniform distribution of microwave heating energy in the food. The benefits achieved by the substitution of organic liquids for water are said to occur because these organic oils have a lower dielectric constant than water and, therefore, absorb microwave energy less readily than water.
It is an object of this invention to provide a means for cooking in a microwave oven at temperatures significantly higher than achieved conventionally. It is another object of this invention to provide liquids for use in preparing comestibles in microwave ovens at temperatures significantly above 100.degree. C.
It is a further object of this invention to provide a process for conducting cooking or reheating in a microwave oven so as to achieve browning and crisping of foods as is conventionally obtained in a thermal oven.